This invention relates to starch compositions useful in forming flexible films. More particularly, it relates to film-forming compositions containing starch in combination with certain plasticizers.
Gelatin is a protein that forms thermo-reversible films. Gel masses composed of gelatin and a plasticizer such as glycerin are formulated to be liquid above room temperature, form a film when cast on a cooled surface, and re-melt when exposed to higher temperatures again. This ability to re-tackify enables encapsulation of liquid materials in gelatin soft capsules. Films formed from plasticized gelatin set very quickly and have high wet film strength. They are also very elastic with good clarity. Plasticized gelatin also has a relatively low viscosity, even when used at high solids concentrations. In addition, when gelatin is in the presence of water at room temperature, it swells but does not go into solution until heat is applied.
Although gelatin is useful in soft gel applications because of its rapid gelling ability, excellent film forming properties, and ability to impart oxygen impermeability, it has the disadvantages of high cost, limited availability, non-kosher status for food products and, at times, batch property variations. Because of these shortcomings, those industries where the need for gelatin is greatest have long sought means for replacing gelatin.
A number of food and industrial applications would benefit by an inexpensive and readily available structural material from a renewable resource, particularly one that is edible and/or biodegradable. Applications such as agricultural mulch, food packaging, and soft and hard gel capsules for cosmetics, pharmaceuticals and paintballs, need a material that is both strong and flexible under a range of use conditions. Applications such as adhesives, coatings, and caulkings do not have the rigorous strength requirements of free-standing films, but lack of brittleness can be just as important.
Starch meets many of the above requirements, and is an attractive raw material for these applications. Its tendency to brittleness, however, has generally blocked its use in these areas.
There have been some previous reports of plasticized starch films. Lourdin (D. Lourdin, G. D. Valle, P. Colonna, Carbohydrate Polymers 27 (1995) 261-270) incorporated up to 20% glycerol in starch films, but found that their mechanical properties were far inferior to those of synthetic film formers. They found that increasing the amylose content of the starch improved tensile strength. Arranitoyannis (Carbohydrate Polymers 36 (1998) 105-119) measured moderately high tensile values on starch films containing up to 25% polyhydric alcohols. When the plasticized films were oven dried to less than 6% moisture content, they became extremely brittle. Incorporating 15% hydroxypropyl units onto the starch molecules improved film flexibility. Shih (F. F. Shih, in xe2x80x9cChemistry of Novel Foodsxe2x80x9d, A. M. Spanier, M. Tamura, H. Okai, 0. Mills, eds., Allured Pub. Corp., Carol Stream, Ill., Ch. 14, pp 179-186) reported tensile values on starch films containing up to 25% plasticizer, but reported that unplasticized films were too fragile for instrumental analysis. In general, increasing plasticizer increases film elongation while decreasing modulus and tensile strength. Other reports of starch-based films include J. D. Christen, U.S. Pat. No. 4,026,986, 1977; J. C. Rankin, I. A. Wolf, H. A. Davis, C. E. Rist, Industr. Engr. Chem. 3 (1958), 120-123; 1. A. Wolff, H. A. Davis, J. E. Cluskey, L. J. Gundrum, C. E. Rist, Industr. Eng Chem. 43 (1951) 915-919; A. M. Mark, W. B. Roth, C. L. Mehltretter, C. W. Rist, Food Technol. 20 (1966), 75-77; W. B. Roth, C. L. Mehltretter, Food Technol. 21, (1967), 72-74; L. Jokay, G. E. Nelson, E. L. Powell, Food Technol. 21 (1967), 1064-1066; and J. L. Willet, B. K. Jasberg, C. L Swanson in xe2x80x9cPolymers from Agricultural Coproducts, eds. M. L. Fishman, R. G. Friedman and S. J. Huange, pp 50-68, Amer. Chem. Soc., Washington DC. These films are reported to be very sensitive to environmental humidity and tend to embrittle in low humidity environments.
Starch has also been included as a component in films containing proteins, such as gelatin, and carbohydrate-based hydrocolloids. Plasticizers are generally added to these systems as well. In most of these systems, starch is a secondary film former; the mechanical properties of the film reflect more strongly the properties of the other polymer(s) in the film. The following illustrates these types of systems. Laurent (L. Laurent, European Patent 0 547 551 Al, 1992), combines 5-40% starch with 5-40% gelatin and 10-40% plasticizer to make flexible, edible films. Arvanitoyannis (I. Arvanitoyannis, E. Psomiadou, A. Nakayama; Carbohydrate Polymers 31, (1996) 179-192) developed edible films containing starch and sodium caseinate with up to 30% sugar or glycerol. Arvanitoyannis (I. Arvanitoyannis, A. Nakayama, S. Aiba, Carbohydrate Polymers 36 (1998) 105-119) developed edible films composed of starch and gelatin with up to 25% polyol plasticizer. Psomiadou (E. Psomiadou, I. Arvanitoyannis, N. Yamamoto; Carbohydrate Polymers 31 (1996) 193-204) studied films composed of starch, microcrystalline cellulose and methylcellulose containing up to 30% polyol plasticizers.
There is a long-standing need for improved film-forming compositions that do not have the shortcomings of prior art compositions.
One aspect of the present invention is a gelatin-free film-forming composition that comprises starch material and a primary external plasticizer. The starch material is selected from the group consisting of modified starch and waxy starch, and has a dextrose equivalent (DE) of less than about 1, and preferably has no measurable DE (using the Lane-Eynon method). The weight ratio of plasticizer to starch material in the composition preferably is at least 0.5:1, more preferably is from about 0.5:1 to about 3:1, and most preferably is from about 1:1 to about 3:1. This composition optionally may include, in addition to starch and plasticizer, gums, hydrocolloids, synthetic polymers, and/or other additives, but is preferably free of protein. xe2x80x9cGelatin-freexe2x80x9d and xe2x80x9cprotein-freexe2x80x9d are used herein to mean that no more than trace amounts (e.g., no more than about 0.1 weight percent) of the listed material is present in the composition.
The composition can be prepared with water, and preferably has a solids concentration of about 30-70%. (All composition percentages given herein are by weight unless otherwise stated.) In one preferred embodiment of the invention, the solids in the composition comprise 25-50% starch material and 50-75% plasticizer.
The starch material preferably comprises starch that has been chemically modified with a monoreactive moiety to a degree of substitution of at least about 0.015. In a particularly preferred embodiment, the starch material is selected from the group consisting of ether and ester derivatives of starch, such as hydroxypropyl, hydroxyethyl, succinate, and octenyl succinate starch. One specific embodiment of the invention comprises hydroxypropylated potato starch having a degree of substitution of about 0.015-0.30 and a molecular weight of about 200,000-2,000,000. Another specific embodiment of the invention comprises hydroxyethylated corn starch having a degree of substitution of about 0.015-0.3 and a molecular weight of about 200,000-2,000,000. Another specific embodiment of the invention comprises hydroxypropylated high-amylose corn starch with a degree of substitution of 0.015-0.3 and a molecular weight of about 200,000-2,000,000.
In some embodiments of the invention, one or more water soluble gums are added to the mixture of starch and plasticizer. The gum is preferably 0-15% of the total solids in the mixture. The gum preferably is selected from the group consisting of carrageenan, locust bean, xanthan, gellan, agar, alginates, guar, gum arabic, and pectin. A combination of kappa carrageenan and iota carrageenan, most preferably in a weight ratio of about 1:1, is especially preferred.
In another embodiment of the invention, water soluble synthetic polymers may be added to the starch and plasticizer mixture. The synthetic polymer is preferably 0-50% of the total solids in the mixture. The preferred synthetic polymer is polyvinyl alcohol.
In another embodiment of the invention, organic or inorganic filler or pigment particles can be added. The pigments may be chosen from a list including clays, calcium carbonate, titanium dioxide, and synthetic organic pigments.
Industrial plasticizers are discussed in the Encyclopedia of Chemical Technology, 4th ed., Vol 19, pp 258-280, 1997. A plasticizer is a substance which, when added to another material, increases the softness and flexibility of that material. Without being bound by theory, it is believed that plasticizers increase flexibility of polymeric materials by increasing the free volume within the material. Randomly distributed within the material and interspersed among the polymer chains, the plasticizer molecules interfere with the polymer""s ability to align its chains and pack into ordered structures. Molecular ordering increases the density of the material (decreases free volume) and impedes mobility of the polymer chains within the material. The increase in free volume imparted by the plasticizer allows room for chain segments to move. The material can then more readily accommodate an applied force by deforming.
Polymers can be plasticized in two general ways: xe2x80x9cinternallyxe2x80x9d and xe2x80x9cexternally.xe2x80x9d Internal plasticization can occur, for example, through the incorporation of a variety of chemical moieties along the starch molecular chains through ether or ester linkages. These moieties include hydroxypropyl, hydroxyethyl, carboxymethyl, succinyl and octenylsuccinyl, to name a few. An irregular array of substituents along the polymer backbone prevents close and regular chain packing, and increases free volume in the material.
External plasticizers are relatively small molecules that are miscible with the polymer, and impede chain alignment. External plasticizers are of two distinct classes: primary and secondary. Primary plasticizers are effective in modifying the mechanical properties of the material on their own. Secondary plasticizers may be incompatible, or ineffective, at plasticizing the material on their own, but when added in combination with the primary plasticizer, can be very effective. They are sometimes called xe2x80x9cextenders.xe2x80x9d
The plasticizers required in this invention are primary, external plasticizers, such as sugars and low molecular weight polyols. The properties of the composition optionally can be further enhanced by using internal and/or secondary external plasticizers. A suitable secondary external plasticizer is water. The polyhydric alcohols are hygroscopic; their presence in the starch compositions increases the water content relative to an un-plasticized starch.
Preferred plasticizers for use in the present invention have the general formula CnOnHx, wherein n has a value between 3 and 6, and x has a value between 2n and (2n+2), where at least 80% of the oxygen is in the form of hydroxyl groups, and the remaining are in the form of ether groups. This group of preferred plasticizers also includes dimers, disaccharides and low molecular weight (e.g., 300-1800 MW) oligosaccarides of these compounds, and may also include ether or ester derivatives of these compounds. Particular examples of suitable plasticizers include glycerol, diethylene glycol, sorbitol, sorbitol esters, maltitol, sucrose, fructose, invert sugars, corn syrup, and mixtures of one or more of these.
In preparing the films described in this invention, the mixture of starch and water is heated with stirring to hydrate fully all components in the mixture. The hydration of starch by heating is termed xe2x80x9ccooking.xe2x80x9d The preferred conditions for cooking the starch mixture are 80-200xc2x0 C. for 5-60 minutes. Those versed in the art of starch cooking will recognize that a variety of cooking techniques may be employed, including but not limited to, open kettles or high-pressure jet cookers. In another embodiment of the invention, instant, pre-gelled or cold-water swelling starches may be used. For these starches, it is not required that the mixture be heated to hydrate fully the starch.
The plasticizer, may be, but is not required to be, mixed with the starch and water prior to cooking. The gum, synthetic polymer, or other components of the film-forming mix, may be, but are not required to be, mixed with the starch and water prior to cooking. Whether or not the non-starch components are mixed with the starch and water prior to cooking will depend on a number of considerations, including the hydration requirements of the other components, their thermal stability, viscosity constraints, and convenience.
Another aspect of the invention is a flexible adhesive for paper tape and other paper-based articles comprised of the above-described starch-based composition, usually with much of the water removed. Yet another aspect of the invention is a flexible paper coating comprised of the above-described composition. Yet another aspect of the invention is a flexible coating in which the starch-based composition is a film-forming component, but which may also include fillers or pigments, latex emulsions or other additives.
Another aspect of the invention is an edible film that comprises the above-described starch-based composition, usually with much of the water removed. Yet another aspect of the invention is a soft gel capsule that comprises a sealed capsule wall and a first substance that is encapsulated by the sealed capsule wall. The capsule wall comprises the above-described starch-based composition. In one embodiment of the invention, the film or the capsule wall consists essentially of the combination of starch material and plasticizer.
The first substance encapsulated by the capsule wall can be any of a variety of materials that have been encapsulated by gelatin in the past. Many such substances are edible, including drugs, vitamins, nutritional supplements, and pre-measured food ingredients such as flavorings. It can also comprise, for example, photographic or dye solutions.
Another aspect of the invention is a method of encapsulating a first substance. This method comprises the steps of providing a first substance and an edible film as described above, and encapsulating the first substance in the film. Preferably, the film used in this method has been formed on a surface having a temperature of at least about 100xc2x0 F.
In one preferred embodiment of the invention, the film or capsule wall consists essentially of the combination of starch material, plasticizer, and optionally gum.
The present invention provides an economical alternative to the synthetic polymers currently used to impart dimensional stability and binding strength in adhesives and industrial coatings. It also provides an economical means for replacing gelatin in compositions utilized in the production of soft gel for food, pharmaceutical, and industrial applications. Further, the starch-based materials of this invention are compatible with existing application equipment used for manufacture of tapes, coated papers, and various products that in the past have been primarily comprised of gelatin.
In compositions of the present invention, the starch, plasticizer, and any other solid ingredients preferably make up from about 30 to 70% by weight of an aqueous slurry. Flexible films are prepared by blending together the starch, plasticizer, and water, and heating the mixture to a temperature and for a time sufficient to gelatinize the starch fully, (e.g., 80-200xc2x0 C. for 5-60 min). Additional materials may be added to the mixture of starch and plasticizer in order to impart improved functionality. These materials may be added before or after heat treatment. The mixture is then sheeted, while warm or hot, to form a thin film. The mixture may be sheeted directly onto paper, board or other surface when used as a coating or adhesive, or onto a casting surface from which the cooled film can be lifted and transfer to rolls, or to fabrication equipment.
The present invention has a number of benefits. Starch is a low cost and readily available material. The starch may be modified using a number of chemical and physical means to enhance its properties while maintaining its status as a material approved as a food additive by the FDA. It may be subjected to a number of additional modifications while maintaining its FDA acceptability for use in contact with foods. It is biodegradable. It is water soluble and therefore does not require expensive, hazardous and/or volatile solvents that many other polymers require for processing. A range of materials are available for plasticizing starch which are both inexpensive and FDA approved for food use. In addition, the compositions of the present invention can be cooked more easily than the high amylose compositions that have been used in the past.
A film comprising the above-described composition can function as a pressure sensitive adhesive. The combination of a high plasticizer content and a highly substituted starch plasticizes the film to the point of providing tack through a broad humidity range.
Examples of modified starches that can be used in the present invention include non-retrograding starches derived by chemical modification of starch from any plant source, including corn, waxy maize, potato, sweet potato, wheat, rice, sago, tapioca, sorghum, high amylose corn, and the like. The particular starch chosen will depend on its performance, availability, and cost. Among the useful modified starches are the common ether and ester derivatives of starch, including but not limited to hydroxypropyl, hydroxyethyl, succinate, and octenyl succinate starch derivatives. Because waxy starches do not retrograde, they are suitable for use without derivatization. Also included among the modified starches suitable for use in the practice of this invention are the thermally converted, fluidity or thin boiling type products derived from the aforementioned types of chemically modified starches. Such materials may be of lower molecular weight, prepared by heating the modified starch alone or by subjecting the starch to a hydrolytic acid and/or heat treatment, or by any other known method designed for the thermal conversion of the starch, such as enzymic heat treatment.
Preferred modified starches are the hydroxyethyl derivatives of dent corn starch and the hydroxypropyl derivatives of potato starch, each preferably having a degree of substitution from 0.015-0.30 ds and a molecular weight of from 100,000 to 2,000,000. In the case of waxy starches of corn, potato, etc., the branches of the amylopectin replace the function of the ether or ester substituents; these starches are functional in the present invention without additional chemical modification, although their properties are not impaired by additional modification, and are enhanced by molecular weight reduction.
Suitable plasticizers include, but are not limited to, glycerol, sorbitol, maltitol, fructose, sucrose, corn syrup, and mixtures thereof.
A variety of optional ingredients may be incorporated into the starch compositions of this invention, before, during, or after cooking the starch. Among the suitable materials that may be utilized are gums, synthetic polymers, preservatives, colorants, clays, pigments, flavoring agents, hardeners, antifoggers, sensitizers, and spreading agents. The inclusion of such additives has no adverse effect upon the properties exhibited by the novel starch-based compositions of the present invention.
Suitable hydrocolloid gums include carrageenan, locust bean gum, xanthan gum, gellan gum, agar, alginates, guar gum, gum arabic, cellulosic derivatives and pectin. Suitable synthetic polymer additives include polyvinyl alcohol, polyethylene glycol, polyacrylamide and polyethylene oxide, and certain derivatives of these polymers.
A composition of the present invention is formed by combining the dry solids (i.e., the modified starch or waxy starch, plasticizer, and any other additives), slurrying in water, and heating at a temperature and for a time sufficient to hydrate the starch, and other gums if necessary. Optionally, this can take place under a vacuum or high pressure. Films can be formed from these starch-based compositions by any conventional method designed to solubilize and deposit a continuous coating or layer of the solution onto a substrate or mold of any form. Among the suitable coating techniques are spraying, dipping, air knife, trailing blade, reverse and direct roll coaters, etc. A film, such as an overcoating or capsule shell, may then be formed by drying the coated solution to a desired moisture content, using any means suitable for the particular purpose. Suitable conventional means include heated rollers, warm or cold air impingement, low humidity chamber or oven drying, etc. For example, during paper coating, the coated sheet is passed over steam-heated rolls to drive moisture from the coating.